New methods for lanthanide sensitization are of great interest due to the incorporation of lanthanide luminescence in a portfolio of applications, including photonic materials, liquid lasers, and telecommunication devices. Near-IR (NIR)-emitting lanthanides are of particular interest for biological imaging because of their advantageous electronic properties compared to that of common organic fluorophores.
Lanthanide emission bands are sharp, occur at fixed wavelengths, and are not affected by environmental factors such as pH or temperature. Accordingly, they are easily detected and discriminated from background signals. Lanthanide cations also have long luminescence lifetimes and are highly resistant to photobleaching, allowing for extended storage time and repeated exposure to excitation sources.
Since f-f transitions are Laporte-forbidden, however, free lanthanide cations have extremely low absorptivity, which limits their luminescence intensity. To overcome this limitation, an “antenna effect” has been exploited to improve the efficiency of lanthanide luminescence. By this approach, lanthanide cations are complexed with “antennae,” i.e., molecules with high absorptivity that can transfer their absorbed energy to sensitize the lanthanide cations.
Different strategies have been applied to sensitize NIR-emitting Ln, including use of dendrimer and nanocrystalline forms. The organization of the antennae about the lanthanide cation significantly impacts the luminescent properties of the complex. Ideally, lanthanides should be effectively shielded from water (O—H) vibrations for sufficient luminescence lifetimes and for discrimination from background signals.
These challenges are hard to overcome because lanthanide cations generally exhibit low stereochemical requirements. Accordingly, it has proven difficult to control the organization of the ligands in a prescribed manner.